Some of it comes from privacy/foss fundamentalism that is very prevalent in fediverse (the entire fedi is basically just a huge nerd circle). People don't really think about more than "Meta bad. Defed". It's a bit of a kneejerk reaction...and it's quite normal around here.
perhaps the goal is simply to split the fediverse into essentially two sides, the Threads side and the non-Threads side
Yes exactly.
I'm unsure how boosts work in this scenario, perhaps those instances are concerned that they'll see Threads content when mastodon.social or other Threads-federated instances users boost it, or that their content will be boosted to Threads users?
Again, spot on.
Consider this:
pure.social blocks evil.meta, but doesn’t block mastodon.social.
A user on pure.social posts something publicly, and since they have a follower on mastodon.social, the pure.social server sends the post to mastodon.social, but doesn’t send it to evil.meta because they're defederated on pure.social.
A user on mastodon.social sees the post and boosts it, and since that user has a follower on evil,meta, the mastodon.social server sends the boost to evil.meta (because evil.meta is not defederated on mastodon.social), and tells the server about the original post from pure.social.
evil.meta receives the boost, and downloads the content of the post from pure.social. pure.social allows evil.meta to download the post because it doesn’t know who is asking.
Beyond the fact that evil.meta was able to see posts from pure.social even though they are defederated, there's also a problem where people on evil.meta start replying to the post - and while the OP on pure.social does not see what they're saying, they might see "half" of the discussion from the replies the user on mastodon.social posts.
This is of course a bit of a moot point, because the OP on pure.social posted it as "public" - and public things on the Internet... well..
I personally think Meta should be banned and regulators should tear the whole company apart. So I'm not too sad about people blocking them. I do think it's a bit premature at this point though. We haven't seen their ActivityPub implementation yet as it didn't roll out with the release version. So I am in the "defederate the shit out of them, but wait and see first"-boat.